1. Field of the Invention
The present invention relates to a method for controlling the communication with mobile stations in a network, preferably in a WLAN (Wireless Local Area Network) according to the IEEE 802.11 standard, wherein the network comprises access points, over which the mobile stations are associated with the network, and wherein the mobile stations can be set to power save mode.
2. Description of the Related Art
Many mobile stations have in most cases wireless access to a network, with which they can, for example, access the Internet or e-mailboxes. The IEEE 802.11 standard represents a widely accepted access technology to wireless local networks (Wireless Local Area Networks, WLANs) and is more and more commonly used in private contexts and hotspots in order to access the internet. In case of mobile stations that are mostly run by power supplies that are independent from the electricity network, as for example by batteries, accumulators, or fuel cells, a utilization of the limited energy available as efficient as possible is particularly important. It is hence requested to de-activate individual functions of the mobile stations whenever they are not needed. Whenever needed, the availability of the functions should not be hampered though. Accordingly, also in case of wireless network accesses the activity of the sending and receiving function of the mobile station can always be reduced, when no IP packets are received or sent by the mobile station for a longer time. At the same time, it has to be ensured that also in power save mode possibly incoming packets can be received.
For this end, the IEEE 802.11 specification shows the basic mechanisms to support the network interface card (NIC) of mobile stations after having entered the power save mode (PSM). Furthermore, the IEEE 802.11 standard allows for a signaling of existent data traffic, as well as for forwarding of IP packets arriving at IEEE 802.11 access points (AP) to mobile stations that have entered PSM. The correspondingly relevant configurations are performed during an association process between the mobile device and an access point. Here, the mobile device gives information to the access point about parameters concerning its listen interval (LI), and in exchange the access point assigns the mobile device an association identifier (AID). The whole control of a mobile station in PSM is hence with the access point, i.e. the access point administers the state of the mobile station, buffers IP packets destined for the mobile station in power save mode, and informs the mobile station by a so-called traffic indication map (TIM) when packets have arrived that can be polled, i.e. collected, from the access point. Finally, the access point forwards the buffered data packets to the corresponding station, when the station retrieves the packets by an IEEE 802.11—specific signaling message (a so-called power save poll, PSP) packetwise.
For communication between a mobile station and an access point the AID acquired during the association process is used. In this context, it is problematic that the AID is only valid as long as the mobile remains associated with this particular access point. If the mobile station moves, a change of the coverage areas of individual access points is rather unavoidable. Consequently, as soon as the mobile station changes into the coverage area of another access point, a new association has to be performed and a new AID has to be requested. If a mobile station is in PSM, it has to leave the PSM in order to newly associate, and can re-enter PSM only afterwards. This leads to unnecessarily high power consumption, as compared to when the mobile station could stay in PSM due to a reduced activity. (See M. Liebsch, X. Perez-Costa, “Utilization of the IEEE802.11 Power Save Mode with IP Paging,” In Proceedings of IEEE International Conference on Communications (ICC), Seoul, Korea, May 2005.)